Respiration in Plants

2.1 Core concepts

• Respiration defined. The breaking of C–C bonds of complex compounds through oxidation within the cells, leading to the release of a considerable amount of energy, is called respiration. The compounds oxidised in the process are called respiratory substrates; usually carbohydrates are oxidised, but proteins, fats and even organic acids can be used as respiratory substrates under certain conditions. The energy is not released in one step but in a series of slow, stepwise reactions controlled by enzymes, and is trapped as chemical energy in the form of ATP — the energy currency of the cell (NCERT §12 intro, p. 154).
• Source of food. Green plants and cyanobacteria prepare their own food through photosynthesis, storing chemical energy in glucose, sucrose and starch; non-green plant cells, animals (herbivores/carnivores) and saprophytes (fungi) ultimately depend on photosynthesis. Photosynthesis occurs in chloroplasts; respiration (breakdown of complex molecules) takes place in the cytoplasm and the mitochondria in eukaryotes (NCERT §12 intro, p. 154).
• Why plants need no respiratory organs. Plants have no specialised organs for gaseous exchange but use stomata and lenticels. Three reasons NCERT gives: (i) each plant part takes care of its own gas-exchange needs with little transport of gases between parts; (ii) plants do not present great demands for gas exchange — roots, stems and leaves respire at rates far lower than animals; (iii) the diffusion distance is small because each living cell is close to the surface, helped further by loose packing of parenchyma that creates an interconnected network of air spaces. Living cells in woody stems remain organised in thin layers inside/beneath the bark, with lenticels as openings (NCERT §12.1, pp. 154–155).
• Combustion vs respiration. The complete combustion of glucose is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy; in combustion most energy is given out as heat. The cell instead oxidises glucose in small steps so that the energy released can be coupled to ATP synthesis (NCERT §12.1, p. 155).
• Anaerobic capability. The first cells on this planet lived in an atmosphere that lacked oxygen; even today some organisms are facultative anaerobes and others obligate anaerobes. In any case, all living organisms retain the enzymatic machinery to partially oxidise glucose without the help of oxygen — this breakdown of glucose to pyruvic acid is called glycolysis (NCERT §12.1, p. 155).
• Glycolysis — site & history. The term glycolysis comes from Greek glycos (sugar) and lysis (splitting). The scheme was given by Gustav Embden, Otto Meyerhof and J. Parnas — hence the EMP pathway. It occurs in the cytoplasm of the cell in all living organisms and is the only respiratory process in anaerobic organisms. Glucose undergoes partial oxidation to form two molecules of pyruvic acid via a chain of ten enzyme-catalysed reactions; in plants this glucose is derived from sucrose (end-product of photosynthesis) or from storage carbohydrates. Sucrose is converted into glucose and fructose by invertase (NCERT §12.2, pp. 155–156).
• Glycolysis — step-by-step. Glucose and fructose are phosphorylated to glucose-6-phosphate by hexokinase; this isomerises to fructose-6-phosphate, then phosphorylates to fructose-1,6-bisphosphate. ATP is utilised at two steps — glucose → G-6-P and F-6-P → F-1,6-BP. Fructose-1,6-bisphosphate is split into dihydroxyacetone phosphate (DHAP) and 3-phosphoglyceraldehyde (PGAL). PGAL → 1,3-BPGA is the step where NADH + H⁺ is formed from NAD⁺ (two redox-equivalents removed as hydrogen atoms). BPGA → 3-PGA yields ATP (substrate-level phosphorylation), and PEP → pyruvate yields a second ATP. Net per glucose: 2 ATP and 2 NADH (NCERT §12.2, p. 156, Fig. 12.1).
• Fates of pyruvate. Three major ways cells handle pyruvic acid: (a) lactic acid fermentation, (b) alcoholic fermentation and (c) aerobic respiration. Fermentation occurs under anaerobic conditions in many prokaryotes and unicellular eukaryotes; complete oxidation of glucose to CO₂ and H₂O requires Krebs' cycle and O₂ (NCERT §12.2 end / §12.3, p. 157).
• Alcoholic fermentation (yeast). Pyruvate → CO₂ + ethanol under anaerobic conditions, catalysed by pyruvic acid decarboxylase and alcohol dehydrogenase. Yeasts poison themselves to death when ethanol reaches ~13 per cent, which limits naturally fermented alcohol; higher-strength beverages are obtained by distillation (NCERT §12.3, p. 157).
• Lactic acid fermentation. Some bacteria produce lactic acid from pyruvate; in animal/muscle cells during exercise, when O₂ is inadequate, pyruvic acid is reduced to lactic acid by lactate dehydrogenase. NADH + H⁺ is the reducing agent and is reoxidised to NAD⁺ in both fermentations (NCERT §12.3, p. 157, Fig. 12.2).
• Why fermentation is inefficient. Less than 7 per cent of the energy in glucose is released, and not all of it is trapped as high-energy bonds of ATP; further, alcohol or acid is hazardous. NADH is oxidised slowly in fermentation but vigorously in aerobic respiration (NCERT §12.3 / §12.5).
• Aerobic respiration — two crucial events. In eukaryotes, the final product of glycolysis (pyruvate) is transported into the mitochondria. The crucial events are: (i) complete oxidation of pyruvate by stepwise removal of all hydrogen atoms, leaving three CO₂ molecules; (ii) passing of electrons removed (as part of the hydrogen atoms) onto molecular O₂ with simultaneous synthesis of ATP. The first event takes place in the matrix of the mitochondrion; the second on the inner membrane (NCERT §12.4, p. 158).
• Link reaction (oxidative decarboxylation of pyruvate). In the matrix, pyruvic acid + CoA + NAD⁺ → Acetyl-CoA + CO₂ + NADH + H⁺, catalysed by the pyruvate dehydrogenase complex requiring several coenzymes including NAD⁺, Mg²⁺ and Coenzyme A. Two NADH are produced per glucose (one per pyruvate) (NCERT §12.4, p. 158).
• Krebs'/TCA cycle — opening. Acetyl-CoA enters a cyclic pathway, the tricarboxylic acid cycle, named after Hans Krebs who first elucidated it. The TCA cycle starts with the condensation of the acetyl group with oxaloacetic acid (OAA) and water to yield citric acid (6C) — catalysed by citrate synthase with release of CoA. Citrate is then isomerised to isocitrate, followed by two successive decarboxylations leading to α-ketoglutaric acid (5C) and then succinyl-CoA (4C) (NCERT §12.4.1, pp. 158–159).
• Krebs cycle — energy harvest. During conversion of succinyl-CoA to succinic acid, a molecule of GTP is synthesised by substrate-level phosphorylation; in a coupled reaction GTP is converted to GDP with simultaneous synthesis of ATP from ADP. There are three points where NAD⁺ is reduced to NADH + H⁺ (at α-ketoglutarate, succinyl-CoA formation, and malate → OAA) and one point where FAD⁺ is reduced to FADH₂ (at succinate → fumarate). Continued oxidation of acetyl-CoA needs continued replenishment of OAA plus regeneration of NAD⁺ and FAD⁺ from NADH and FADH₂. Summary per pyruvate: Pyruvic acid + 4 NAD⁺ + FAD⁺ + 2 H₂O + ADP + Pi → 3 CO₂ + 4 NADH + 4 H⁺ + FADH₂ + ATP (NCERT §12.4.1, p. 159, Fig. 12.3).
• Pre-ETS audit per glucose. Glucose has now been broken down to release CO₂ and eight molecules of NADH + H⁺ and two of FADH₂ synthesised, besides just two ATP from the TCA cycle (NCERT §12.4.1, p. 159).
• ETS location and carriers. The electron transport system (ETS) is present in the inner mitochondrial membrane. Electrons from NADH produced in the mitochondrial matrix are oxidised by NADH dehydrogenase (Complex I) and transferred to ubiquinone (UQ) located within the inner membrane. Ubiquinone also receives reducing equivalents via FADH₂ (Complex II — succinate dehydrogenase) generated during succinate → fumarate. Reduced ubiquinone (ubiquinol, UQH₂) is then oxidised with transfer of electrons to cytochrome c via cytochrome bc₁ complex (Complex III). Cytochrome c is a small protein attached to the outer surface of the inner membrane and acts as a mobile carrier between Complex III and Complex IV. Complex IV is the cytochrome c oxidase complex containing cytochromes a and a₃ and two copper centres (NCERT §12.4.2, pp. 159–160, Fig. 12.4).
• Oxidative phosphorylation & ATP yields. Electron passage through Complexes I to IV is coupled to ATP synthase (Complex V) for ATP production from ADP and inorganic phosphate. Oxidation of one NADH gives rise to 3 ATP; oxidation of one FADH₂ produces 2 ATP. O₂ is the final hydrogen acceptor only at the terminal step but is vital because it drives the whole process by removing hydrogen from the system. The energy of oxidation–reduction (not light) drives ATP formation; hence the name oxidative phosphorylation (NCERT §12.4.2, p. 160).
• ATP synthase / chemiosmosis. Complex V consists of F₁ (peripheral headpiece, contains site for ATP synthesis from ADP + Pi) and F₀ (integral channel through which protons cross the inner membrane). Passage of protons through F₀ is coupled to the catalytic site of F₁ for ATP production. For each ATP produced, 4H⁺ pass through F₀ from the intermembrane space to the matrix down the electrochemical proton gradient (NCERT §12.4.2, p. 161, Fig. 12.5).
• Respiratory balance sheet — net 38 ATP. Net gain calculations require four idealised assumptions: (i) sequential, orderly pathway functioning — glycolysis, TCA and ETS one after another; (ii) NADH synthesised in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation; (iii) none of the intermediates is utilised to synthesise any other compound; (iv) only glucose is being respired — no other substrate enters at intermediate stages. Under these assumptions there is a net gain of 38 ATP molecules during aerobic respiration of one molecule of glucose (NCERT §12.5, p. 161).
• Fermentation vs aerobic — three differences. (a) Fermentation = partial breakdown; aerobic = complete to CO₂ and H₂O. (b) Fermentation = net 2 ATP per glucose; aerobic = many more. (c) NADH is reoxidised slowly in fermentation, very vigorously in aerobic respiration (NCERT §12.5, p. 162).
• Amphibolic pathway. Glucose is the favoured substrate; all carbohydrates are first converted into glucose. Fats are broken into glycerol + fatty acids first; fatty acids degrade to acetyl-CoA and enter the pathway; glycerol enters as PGAL. Proteins are degraded by proteases and individual amino acids (after deamination) enter at pyruvate, acetyl-CoA, or various Krebs intermediates depending on structure. Since the same intermediates are also withdrawn for biosynthesis of fatty acids and amino acids, the respiratory pathway is involved in both catabolism and anabolism and is best called an amphibolic pathway rather than a purely catabolic one (NCERT §12.6, p. 162, Fig. 12.6).
• Respiratory Quotient (RQ). RQ = volume of CO₂ evolved / volume of O₂ consumed in respiration; it depends on the type of respiratory substrate (NCERT §12.7, p. 163).
• RQ values. Carbohydrates (e.g. glucose) — RQ = 1.0 because equal CO₂ and O₂ are exchanged. Fats (e.g. tripalmitin, 2 C₅₁H₉₈O₆ + 145 O₂ → 102 CO₂ + 98 H₂O) — RQ = 102/145 ≈ 0.7. Proteins — RQ ≈ 0.9. In living organisms substrates are usually mixed; pure proteins or fats are never used as respiratory substrates (NCERT §12.7, pp. 163–164).

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Fig. 12.1, p. 156 — Steps of glycolysis. Glucose (6C) → G-6-P → F-6-P → F-1,6-BP → splits into DHAP + PGAL (3C each) → 1,3-BPGA → 3-PGA → 2-PG → PEP → pyruvate (3C). ATP used at steps 1 (hexokinase) and 3 (PFK). NADH+H⁺ formed at PGAL → BPGA. ATP produced at BPGA → 3-PGA and at PEP → pyruvate. Net: 2 ATP and 2 NADH per glucose; H₂O is released at the 2-PG → PEP step (enolase).
• Fig. 12.2, p. 157 — Anaerobic respiration. Glucose → PGAL → 3-PGA → PEP → pyruvate (with NADH+H⁺ generated). Pyruvate branches: (i) to lactic acid (NADH → NAD⁺ via lactate dehydrogenase) or (ii) to ethanol + CO₂ (NADH → NAD⁺ via pyruvic acid decarboxylase + alcohol dehydrogenase). The reducing agent NADH is reoxidised to NAD⁺ in both pathways.
• Fig. 12.3, p. 159 — Citric acid cycle. Pyruvate (3C) + NAD⁺ + CoA → Acetyl-CoA (2C) + CO₂ + NADH+H⁺. Acetyl-CoA + OAA (4C) → Citric acid (6C) → α-ketoglutaric acid (5C) [CO₂ + NADH+H⁺] → Succinyl-CoA (4C) [CO₂ + NADH+H⁺] → Succinic acid (4C) [GTP] → Fumaric → Malic → OAA [FADH₂ released at succinate → fumarate; NADH at malate → OAA].
• Fig. 12.4, p. 160 — ETS. Inner mitochondrial membrane showing intermembrane space (left), inner membrane (centre, with proton arrows 4H⁺ at Complex I, 4H⁺ at Complex III, 2H⁺ at Complex IV) and matrix (right). NADH+H⁺ → Complex I (FMN, Fe–S clusters) → UQ; FADH₂ from succinate → fumarate enters via Complex II (Fe–S, FAD) → UQ. UQH₂ → Complex III (cytochrome bc₁, cyt b, Fe-S, cyt c₁) → cytochrome c (mobile) → Complex IV (Cyt a-Cyt a₃-Cu_A/Cu_B) → ½O₂ + 2H⁺ → H₂O. ATP synthase shown with F₀ embedded and F₁ projecting into matrix; ADP + Pi → ATP, with electrochemical gradient noted.
• Fig. 12.5, p. 161 — ATP synthase. F₀ channel embedded in inner membrane (outer side ↔ matrix); F₁ headpiece projecting into matrix; 4H⁺ flow inward through F₀ per ATP synthesised from ADP + Pi.
• Fig. 12.6, p. 163 — Amphibolic interrelationships. Three substrate boxes at top — Fats, Carbohydrates, Proteins — feed in: fats → fatty acids and glycerol; carbohydrates → simple sugars (e.g. glucose) → G-6-P → F-1,6-BP → DHAP ⇌ PGAL → pyruvic acid → acetyl-CoA → Krebs' cycle → CO₂ + H₂O; proteins → amino acids enter at pyruvic acid / acetyl-CoA / Krebs intermediates.

2.4 Common confusions / NTA trap points

• 2 vs 4 ATP in glycolysis. Four ATP are synthesised (at BPGA → 3-PGA ×2 and PEP → pyruvate ×2) but two are consumed at the start (hexokinase and PFK steps), so the net gain is 2 ATP. NTA loves to ask "net" vs "gross."
• GTP vs ATP in Krebs. Substrate-level phosphorylation in the cycle produces GTP at succinyl-CoA → succinic acid; GTP is then converted to ATP in a coupled reaction — both phrasings are correct, but the direct product is GTP.
• NADH = 3 ATP, FADH₂ = 2 ATP. This is the NCERT-stated ratio; do not use 2.5/1.5 values from outside sources for CUET.
• Site confusion. Glycolysis — cytoplasm; link reaction + Krebs — mitochondrial matrix; ETS + ATP synthase — inner mitochondrial membrane. Cytochrome c sits on the outer surface of the inner membrane (not in the intermembrane space proper).
• 38 ATP is theoretical. NCERT explicitly calls it an idealised number based on four assumptions; questions may quote either "38 ATP" or values like ~36 ATP — pick the one matching the assumptions stated.
• RQ trap. Carbs = 1.0, fats ≈ 0.7, proteins ≈ 0.9. NCERT gives only the tripalmitin worked example (102/145 = 0.7) — match the exact arithmetic if numbers are given.
• Complex II does not pump protons. Only Complexes I, III and IV pump protons; that is why FADH₂ (which enters at II) yields fewer ATP than NADH.
• Alcohol limit in fermentation. Yeasts poison themselves at ~13 per cent ethanol — natural fermented beverages cannot exceed this; higher strengths are obtained by distillation.
• Glycolysis is universal. Glycolysis occurs in all living organisms — not only in anaerobes; in anaerobes it is the only respiratory process.
• Pyruvate fates are three. Lactic acid fermentation, alcoholic fermentation, aerobic respiration — not two.
• Cytochrome c is not free in cytoplasm. It is a small protein attached to the outer surface of the inner mitochondrial membrane.
• Final acceptor is O₂. Oxygen acts as the final hydrogen/electron acceptor at Complex IV; role is "limited to the terminal stage" but drives the whole process.

2.5 Key processes / classifications